1. Field of the Invention
The invention relates to a process for the production of a melamine resin coated paper for the formation of hot pressed, scratch resistant surface layers on laminate synthetics and wooden working materials, such as wood fiber boards, wood chipboards, plywood or similar materials, whereby the paper is preimpregnated with a urea resin which is water soluble in its still unhardened state or with an aminoplast rich in urea and which is water soluble in the still unhardened state, is dried and is provided furthermore at least on one side with an application of a melamine resin which is water soluble in the unhardened state which is also dried and which in the course of the hot pressing process in the case of which the paper is combined with the substrate surface to be coated, will form a cured, scratch resistant chemically constant closed surface layer while flowing.
2. Description of the Prior Art
Melamine resin coated papers, the melamine resin coating of which in the course of the hot pressing process in case of which the paper is connected with the substrate surface that is to be coated, will form a hardened surface layer while flowing. They are widely used for the production of surface coatings, whereby they are used above all in connection with layered molding substances which are built up from phenol resin paper, as well as for the production of cover layers or wood fiber boards and wood chipboards, whenever decorative surface layers are to be created. By the use of such melamine resin coated papers, one will obtain surface layers which are easily scratch resistant as against the customary stresses which occur in the household or in the office and are also capable of easily withstanding the thermal or chemical strains occurring in this area. In this connection, we may mention that various other papers and foils are used for the surface coating, the surface of which will suffer from such stresses and which are considerably inferior especially in regard to the scratch resistance of the surface layers produced on the basis of melamine resin coated papers.
The use of the melamine resins which, as has been mentioned, results in surfaces capable of dealing with a high stress and which moreover, because of the extraordinary optical characteristics of these resins, offers advantages, does represent a considerable cost factor in the case of the production of the coated papers. A substitution of the melamine resins by urea resins, which are available at a more favorable price and which likewise have good optical characteristics, encounters great difficulties as a result of the sensitivity to moisture of hardened urea resins. In the case of use of mixtures of urea resins and melamine resins for the production of surface layers of the type in question here, it will result in an important drop in quality as compared to surface layers which are formed on the basis of pure melamine resins.
In order to substantially lower the costs for the resin in the case of the production of melamine resin coated papers while maintaining the cover layers made of pure melamine resin, known proposals have provided for subjecting the papers first of all to a preimpregnation with a urea resin or with a urea-melamine mixed resin and to equip the papers impregnated thus subsequently with cover layers of melamine resin. In that case, the problem arises that one must effectively counteract a diffusion of the urea resin in the melamine resin cover layer, in order to restrain a deterioration of the quality of the cover layer. Difficulties resulted both in regard to this problem as well as in regard to the processes of saturation or coating and drying of the paper that was to be coated during the practical execution of the known processes. This militated against the general introduction of these processes, whereby frequently a strong tendency for the intermixing of the impregnation resin with the melamine resin cover layer was exhibited.